A number of structures are formed of a plurality of layers that are stacked or otherwise laid one upon another to form a multi-layer structure. For example, air vehicles, such as aircraft, commonly include multi-layer structures to form various portions of the airframe. The multiple layers of a structure may be held together by one or more fasteners. In order to install a fastener, a hole may be drilled or otherwise formed through the multiple layers with a fastener thereafter being inserted through the hole. However, various anomalies may occur at or near the holes through which the fasteners extend, such as fatigue cracking due to the stress concentrations that may be present proximate the holes.
As a result of the multi-layer structure, at least portions of one or more of the layers that are proximate the hole through which the fastener extends may be hidden from view. For example, an intermediate layer of a multi-layer structure may be sandwiched between two outer layers which serve to hide the inner layer from view. By way of example, an air vehicle may include brackets mounted on longerons. The longerons may be mounted on bulkhead flanges which may, in turn, be covered by a skin. Each of these layers may be held together by one or more fasteners extending through holes that have been formed in the layers, e.g., the brackets, the longerons, the bulkhead flanges and the skin. As a result of the stacking of the various layers, portions of at least the longerons and the bulkhead flanges may be hidden from view.
Maintenance and repair personnel may desire to inspect a structure, such as an air vehicle, to identify anomalies such that the anomalies may be evaluated for repair, replacement or the like. However, the hidden layers of a multi-layer structure may not be visibly inspected and, instead, the multi-layer structure may need to be disassembled in order to view the hidden layers, such as to identify any anomalies that occurred at or near a hole through which a fastener extends. Such disassembly may be relatively labor intensive and may increase the cost and the time requirements for the inspection. Additionally, the reassembly of the structure following the inspection may also be problematic in that damage to the structure may be incurred during the reassembly process or portions of the structure may be reassembled incorrectly.
In an effort to facilitate the inspection of various structures, a variety of non-destructive evaluation (“NDE”) techniques have been developed in which signals are transmitted into a structure with the signals that emerge from the structure then being evaluated in order to detect anomalies within the structure. However, NDE techniques may have difficulty in detecting anomalies in the hidden layers of a multi-layer structure due to the deleterious affect upon the signals that is created by the multiple interfaces between layers of the multi-layer structure. In addition, the thickness of a multi-layer structure may also adversely impact the ability to detect anomalies with NDE techniques where the anomalies lie within the hidden layers of the multi-layer structure.
As such, it may be desirable to provide an improved technique for inspecting the hidden layers of a multi-layer structure.